A longstanding issue in hearing research is the "dynamic range problem," the sharp contrast between the wide dynamic range of hearing (>100 dB between threshold and uncomfortable loud sounds) and the narrow dynamic range of most auditory neurons (20-40 dB). We recently found that primary auditory neurons show a novel form of adaptation, distinct from classic firing rate adaptation, whereby the dynamic range adapts to the mean sound level in a continuous, dynamic stimulus. By allowing neurons to respond adaptively and dynamically to high sound levels at which the firing rates would otherwise be saturated, dynamic range adaptation improves the precision of neural coding for the prevailing sound levels in the acoustic environment. The overall goal of the proposed research is to investigate the neural mechanisms underlying dynamic range adaptation through single-unit recordings from auditory neurons presented with continuous, dynamic stimuli. The proposal has three specific aims: (1) Evaluate the role of the cochlear amplifier in dynamic range adaptation by comparing the adaptation in auditory nerve (AN) fibers of cats produced by tones at the characteristic frequency (CF) with that at a frequency well below the CF;(2) Assess the contribution of the synaptic ribbon to dynamic range adaptation through recordings from AN fibers of mutant mice in which the synaptic ribbon is no longer anchored to the presynaptic membrane of the inner hair cell;and (3) Investigate the variability in dynamic range adaptation in the cat cochlear nucleus (CN) by measuring the strength and the time course of dynamic range adaptation of CN neurons of various unit types. Studying dynamic range adaptation and its underlying mechanisms will provide fundamental understanding of the adaptive processing of sound levels in natural acoustic environments, and has important implications for understanding the neural coding of natural sounds such as speech in age-related and noise-induced impaired hearing. PUBLIC HEALTH RELEVANCE: Dynamic range adaptation enables auditory neurons to shift their dynamic range adaptively toward the prevailing sound levels in the stimulus, which extends the narrow dynamic range of single neurons thereby alleviating the longstanding dynamic range problem-the substantial difference between the neural and behavioral dynamic range. Our studies of dynamic range adaptation and its underlying mechanisms will help us gain fundamental understanding of the adaptive processing of natural sounds in the auditory pathway. Our investigation of dynamic range adaptation in mutant mice as a model for impaired hearing may help improve the design of the next-generation adaptive processors for hearing aids and cochlear implants, thereby enhancing the speech intelligibility of hearing impaired listeners in everyday acoustic environments.